generator faults

[gliderman]

Hi, and greetings from the great white north (AKA Canada!). I am flying a 1951 170A with a C-145. Yesterday while enroute I had the misfortune of losing electrical power from the generator. The battery held up long enough to get into controlled airspace and land, but I am wondering if any members might suggest a cure for the problem. Is is the generator, or perhaps the volatge regulator. Any help greatly appreciated.

[Tom Downey]

Disconnect the field wire on the generator, leave it off, make a jumper wire that can connect the battery positive to the generator field post.
connect one end of the jumper wire to the battery positive post, start the engine, while at idle touch the jumper to the field post of the generator, while watching the amp meter.

If the Amp metter pegs to the charge side of the meter, replace the regulator.

If not, replace the generator.

Touch the jumper to the field post only momemtarily. or the smoke gets out.

[GAHorn]

Gliderman, don't do that (what Tom says. He's thinking incorrectly. He doesn't do that often...but this time he's got his electrons backwards. Generator Fields should never be hooked up directly to a positive post of a battery. Fields are always GROUNDED circuits. (Hooking a battery up to it may reverse it's polarity.) The Generator ARMature terminal connects to the battery via the voltage regulator. The Generator FieLD terminal connects to the regulator FieLD terminal thru cockpit master switch. Inside the regulator it is regulated to ground.)

Before doing ANY thing to your system inspect the wiring connections and determine your fuse/circuit breaker is good. Then Re-Polarize your generator/regulator by A) Turn your Master Switch - ON, B) WITHOUT running the engine BRIEFLY touch a jumper wire to the regulator BATtery terminal and the regulator ARMature terminals. Then without turning the Master off, start your engine and run it at 1800 rpm to see if your generator is back online. If so, you're finished.


If still no charge, then hook up a small jumper wire between your Generator field terminal and clip the other end directly to a good airframe ground. If the generator now charges, the problem lies with the regulator or ships wiring. If it doesn't charge the generator is at fault.
The Gen Field Post (this is the small one on the Gen) can remain hooked up just exactly like normal. All you should do is clip a jumper wire onto it and clip the other end of the jumper to a good airframe ground, and start the engine and see if it charges. If it does produce a charge...then your Generator is fine and healthy. This jumper wire has bypassed the master switch and regulator completely. The problem lies either in the regulator or in the aircraft wiring or cockpit master switch. Remove the jumper from aircraft ground (leaving it hooked up to the Gen FieLD terminal, and hook the other end up directly to the regulator FieLD terminal. If the system now charges...the problem lies in the ships wiring or in the cockpit master switch itself.

If it doesn't, ...If it still doesn't produce a charge, then leave one end clipped your regulator FieLD terminal and the other end of the wire to a good airframe GROUND. Run the engine at 1800 to see if the generator charges. If so, then the regulator is bad. If it doesn't then your generator is bad.

For a more detailed description of the generator charging system, and the entire Cessna 170 electrical system, including Troubleshooting and wiring diagrams, contact TIC170A headquarters@cessna170.org and ask Velvet to sell you the Cessna 170 Electrical Systems Service Manual. $20 cheap (member price).

[Tom Downey]

My way produces a momentary over voltage and wll peg the amp meter for a second, if the generator is any good.

And It will not reverse the polarity, Its the same as flashing the field.

[jims170]

There are two ways a generator can be wired, with the field circuit internal grounded or external grounded. The ones on our aircraft are external grounded through the voltage regulator. You can verify which type of generator you have by removing both wires and check for continuity with an OHM meter. When you have continuity between the two terminals it has the externaly grounding windings. The way this works in side the generator is the field power (volts x amps) is tapped off at the Armature lead ( A+, the larger terminal) then run through the windings to create a magnetic field which will create the needed amperage output.
To restore the residual magnetism in the Pole Pieces (which have the field windings wrapped around them) current MUST flow in the correct direction. The correct direction for the current flow is the NORMAL direct for the current when the generator is operating. So current must come OUT the field connection and thus IN the ARM terminal to FLASH the field.

So the short answer is George is correct.

BUT, George's method of flashing the field assumes a good grounding circuit is present though the master switch and on to the voltage reulator. In order to restore the residual magnetism you must have CURRENT FLOW when you flash the field. You can bypass the rest of the circuit by connecting a jumper wire from the field to ground.

If you had the other type of generator, the power for the field is tapped off the inside of the voltage regulator before the reverse current relay and routed through the control circuits then back to the generator field winding to ground inside the generator. Thus, on this type, power would go in the Field terminal to FLASH the field. As you can see, this type of generator would not have continuity between the two terminals as the other one did.

Jim
Aircraft electrician, A&P IA FCC Lic. Avionics / electrical degree......ect

PS, Dont flame me as this is my first attempt to make a contribution

[Bruce Fenstermacher]

PS, Dont flame me as this is my first attempt to make a contribution

Thanks Jim and welcome aboard. Wondering how long you've been lurking.

Flaming? Is that what we do here.

[johneeb]

Jim,
Thanks for the easy to understand explanation of the two types of internal Generator circuits.
Johneb

[zero.one.victor]

Gliderman, if your generator system hasn't been going downhill but just suddenly gave up the ghost, I'd bet it's a broken wire &/or faulty wire connector. This has happened to two friends lately: one had a broken brush wire inside the generator, the other had corrosion where the connector crimped onto the wire at the generator's armature post.
Just take your time & check out all the connections carefully, maybe take them loose & clean them up with Scotchbrite or whatever. Wouldn't hurt to take your battery & starter cables loose & clean them up too.
BTW, I noticed that my starter was turning things over kinda slow a while back, in spite of a fairly new battery. I took the switch (button) on the starter apart & the heavy copper contacts inside were burned & real rough looking. I dressed them up & it made a workld of difference.

Eric

[GAHorn]

There are two ways a generator can be wired, with the field circuit internal grounded or external grounded. The ones on our aircraft are external grounded through the voltage regulator. You can verify which type of generator you have by removing both wires and check for continuity with an OHM meter. When you have continuity between the two terminals it has the externaly grounding windings. The way this works in side the generator is the field power (volts x amps) is tapped off at the Armature lead ( A+, the larger terminal) then run through the windings to create a magnetic field which will create the needed amperage output.
To restore the residual magnetism in the Pole Pieces (which have the field windings wrapped around them) current MUST flow in the correct direction. The correct direction for the current flow is the NORMAL direct for the current when the generator is operating. So current must come OUT the field connection and thus IN the ARM terminal to FLASH the field.

So the short answer is George is correct.

BUT, George's method of flashing the field assumes a good grounding circuit is present though the master switch and on to the voltage reulator. In order to restore the residual magnetism you must have CURRENT FLOW when you flash the field. You can bypass the rest of the circuit by connecting a jumper wire from the field to ground.

If you had the other type of generator, the power for the field is tapped off the inside of the voltage regulator before the reverse current relay and routed through the control circuits then back to the generator field winding to ground inside the generator. Thus, on this type, power would go in the Field terminal to FLASH the field. As you can see, this type of generator would not have continuity between the two terminals as the other one did.

Jim
Aircraft electrician, A&P IA FCC Lic. Avionics / electrical degree......ect

PS, Dont flame me as this is my first attempt to make a contribution

Jim! WE don't have any flaming flamers here! We just have lots of close friends who sometimes forget how the cyberspace word can be misunderstood. I know. Because I started that club!

WELCOME! Glad you're with us. And I'm glad you made the mention of the possibility of a defective ground thru the master switch preventing proper polarizing during flashing of the field. That's a great catch.
I'm out of town and away from my regular resource materials when I wrote that response, so I began to question my memory somewhat.
So, when I was able, I reviewed the 170 Service Manual troubleshooting section to see if I'd had that oversight during creation of that document. I sorta did a little of both. It's stated during the troubleshooting that the aircraft wiring/master switch circuit could be the problem...and grounding the gen-field is performed during the troubleshooting...but...not in a sequence/order that would remove that possibility that such a problem might defeat the polarizing manuever. Thanks for seeing that possibility and bringing it up.
I'll issue a revision to the manual to change the order of the troubleshooting so that the field is grounded manually before flashing the field to overcome that deficiency.

As for comment regarding the internally-grounded generator possibilities ...("If you had the other type of generator...")...I don't believe any generators with internally grounded fields were ever approved for Cessna 170 aircraft. So, while that type generator/regulator system may exist in other applications, (usually automotive, right?) and although informative from the perspective of repairing other equipment...I wouldn't want anyone in this group to think they would use the procedure for that type generator.

Again, thanks for speaking up. PLease keep doing it. We need everyone's expertise!

[jims170]

As for comment regarding the internally-grounded generator possibilities ...("If you had the other type of generator...")...I don't believe any generators with internally grounded fields were ever approved for Cessna 170 aircraft. So, while that type generator/regulator system may exist in other applications, (usually automotive, right?) and although informative from the perspective of repairing other equipment...I wouldn't want anyone in this group to think they would use the procedure for that type generator.


George is correct that all of the Delco generators use an external grounding field circuit which grounds inside the voltage regulator.
In the automotive world both types are commonly used. General Motor (Delco Remy was a former division of GM) did it one way and Ford did it the other way. My point in bring up both types is so you will know that all generator are not the same and do not use the same troubleshooting techniques. So when your buddy say he just got done fixing his generator on his ford tractor and wants to help you fix your airplane generator,,, well you might be better off buying the Cessna 170 manual and doing it yourself.

Jim

[Roesbery]

The first item I'd check is continuity across the field circut of the master switch. these switches have a bit of age on them and a thin layer of corrision or old dried up lubricant on the contacts can prevent contact. Sometimes moving the switch knob a little back and forth will cause the switch to make contact and restore gen function while in flight. If the switch works ok then go to the other checks.

[GAHorn]

The first item I'd check is continuity across the field circut of the master switch. these switches have a bit of age on them and a thin layer of corrision or old dried up lubricant on the contacts can prevent contact. Sometimes moving the switch knob a little back and forth will cause the switch to make contact and restore gen function while in flight. If the switch works ok then go to the other checks.

Yeah, that's a good one too! I took off several years ago to find my gen not "gennin'"... and by pulling firmly on the cockpit master switch and "wiggling" it found the gen to come online. When the flight was over, I removed the switch, pryed the fingers apart to open the little box, and found the contacts exactly as you described. A little sandpaper, a blob of solder to provide a new contact surface, some lithium-based grease, and bending the little fingers back where they held the phenolic cover back onto the "box" and...voila! an "overhauled" cockpit master switch. Five years later....still working perfectly. (But this switchi is still available relatively inexpensively, from several sources. A seemingly identical switch is also sold for automotive purposes at NAPA. (Non-Aircraft Parts Agents)

[cessna170bdriver]

George,

Thanks for the switch "overhaul" procedure. Is it spelled out in the elecrical chapter of the service manual in more detail? Specifically, where does the lithium grease go?

I'm having a similar problem with my nav light switch ('55 B). It often takes an extra tug and wiggle to get the lights to come on.


Miles

[GAHorn]

That procedure is not detailed in the service manual.
The way those switches work, ...the knob is screwed onto a threaded shaft which is connected to a phenolic slide within the "box". The slide has contacts on it which bear against contacts connected directly to the terminals of the switch. The contact points are held against each other by an internal spring arrangement. ("When you take the switch apart, be vewwy vewwy caaful!" -Elmer Fudd)
Use alcohol or electronics cleaner to clean up the old grease and use fine sandpaper to brighten up the contacts. (Don't get carried away...you don't want to reduce the size of the contacts appreciably. You just want to get rid of tarnish/corrosion/dried-grease. Dried grease can be hardened and can contribute to the problem of low contact pressures.) If the contacts are especially worn down, they may be built up somewhat by skilful soldering. The switches really are a simple device internally.
Many electronics stores, and automotive supply houses/parts houses have aerosol cans of White Lithium Grease, which is intended for electrical contacts. (A little Dab'l do ya'."-Beryl Creem) Put the grease directly inside the box and on the springs and contacts/sliders. Reassemble the switch and bend the tabs back down. Make sure you log your certified repair station ID in the logbooks.

[Roesbery]

You can buy special grease for electrical use, But plain old vaselene will work great. I use it on the boat in light sockets to prevent the bulbs from corrroding in place and the wire ends, terminals etc.